The number of installments varies but your options will be made clear to you on the retailers website before you complete your purchase.
You cannot change the number of installments once you have made your selection and your plan is in progress.
In addition to your monthly installment payments, the full outstanding amount is authorized (held) on your card to guarantee future payments. Authorizations are usually renewed every 17-21 days until your plan is paid off. Authorizations get smaller with each installment paid. The previous authorization is released once a new one is obtained. Authorizations show as 'pending' or 'reserved' on your account. If your plan is kept on track, these authorizations are not charged.